The classic rubber band gun. They have been around for a long time an can be made from almost anything. Basically they are all about releasing a rubber band by a trigger, down a stick. You can make them as pretty (or as ugly) as you want but other than looks they are differentiated by their release mechanisms.

Basic - The Peg This is a pretty simple one that anyone can make with a peg, stick and some nails or glue. Attach the peg to the stick (add a handle if you want). Then to load, clip your rubber band in the peg and

stretch over to the end of the stick. To fire, aim and the press the peg.

Other peg method, not so common but most say better, is to have the peg when pressed push the band off a nail. You drill a hole through the end of the peg and put a nail through it so that it sticks out when the peg is not pressed and doesn't when you press the peg. To fire it, the rubber band sits over the nail and when you press it the 'floor', the rubber band is against, rises and the band is released.

More Complex - The Cog Wheel

This one is more complex. You can stretch one (or more) rubber band from the end over one of the cogs in the wheel. Pulling the trigger will let the wheel slip one place, releasing a rubber band.

There are also versions where it doesn't just let one cog go but just release the wheel all together, releasing all the bands at once.

The Rifle

Rifles don't normally have different mechanism, just different shapes. Quite often they boast longer distances and better accuracies though.